Summary

Z-90 activates in July and December if an infected file is executed and the time is exactly 09:03. At this time the virus encrypts the DOS boot sector of drive C:, making the PC unbootable (encryption is done by XORing with 16-bit hex value 0903). After this it displays this message:

        VIRUS Z-90. JUNIO 95. M
        EXICOSE MURIO TU DISCO
        DUROTU LO PUEDES RE
        VIVIRHAS BAILADO ALGUNA VEZ CON TU NOVIA A LE TENUE LUZ DE LA

Z-90 was found in the wild in Brazil in April 1996.

[Analysis: Peter Szor, F-Secure, 1996]